My first three plants were all done in amended Coco...no issues whatsoever. Pulled five ounces off the first auto, and 12 off the second two.
First plant
Second two
I didn't Treat it any different to a peat mix. 70/30 Coco perlite, nothing but 11:2:14 osmocote prills and cannazym In the first, and the second two had organic 3:1:4 and Neem/kelp...top dressed every two weeks once flower started and watered as usual. That run also used cannazym.
Coco and peat are basically both inert medium bases.. the only difference is their buffering capacity and cation exchange capacity. We used dry amended Coco for years at our nursery and had no issues growing tens of thousands of plants. Most people haven't even realised that most potting mixes have moved from peat to coir for sustainability reasons, and the performance usually increases with the introduction of coir instead of peat.
This black and white "you can't amend Coco" is just juvenile. If I was a straight newbie and I managed fine, it can't be that hard.
Forgot.to mention the second two had compost in with the Coco 40 Coco, 30 compost 30 perlite.
This auto was done In amended Coco and peat mix (40 Coco, 30 peat 30 perlite) as well, and it nearly filled a 2x4, pulling over 7 ounces.
these mediums all acted like any peat based potting mix, albeit regulating pH better as peat is acidic.